ObjC: Always dispatch async in UIApplication status observer.

Fixes a possible deadlock.

BUG=webrtc:8130

Review-Url: https://codereview.webrtc.org/3003633002
Cr-Original-Commit-Position: refs/heads/master@{#19464}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: cca0006e33c106a7f62df1a1b50f84676cc5ed1e
1 file changed